The connection between language processing and combinatorics on words is natural. Historically, linguists actually played a part in the beginning of the construction of theoretical combinatorics on words. Some of the terms in current use originate from linguistics: word, prefix, suffix, grammar, syntactic monoid... However, interpenetration between the formal world of computer theory and the intuitive world of linguistics is still a love story with ups and downs. We will encounter in this chapter, for example, terms that specialists of language processing use without bothering about what they mean in mathematics or in linguistics. This chapter is organized around the main levels of any language modeling: first, how words are made from letters; second, how sentences are made from words. We will survey the basic operations of interest for language processing, and for each type of operation we will examine the formal notions and tools involved.